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Altera_Forum's avatar
Altera_Forum
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15 years ago

2.5VCCIO and 1.8V logic

Hi,

I am designing a board using the EP3C40F324C6N device.

The A/D converter that I am using has an LVDS data bus and 1.8V control logic. If I have VCCIO connected to 2.5V (which is required for LVDS), can the same IO bank accept 1.8V logic?

The app notes that I have found all deal with the incoming logic voltage being higher than VCCIO not lower than VCCIO.

Thanks

Bob

3 Replies

  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    15 years ago

    --- Quote Start ---

    If I have VCCIO connected to 2.5V (which is required for LVDS), can the same IO bank accept 1.8V logic?

    --- Quote End ---

    It could work but you have to keep a few things in mind:

    - Like Ironmoose wrote: be aware that the output will be 2.5V

    - The minimum Vih of a 2.5V bank is 1.7V

    - The Voh of your external device will be close to 1.8V in practice, but your datasheet will probably specify 0.7*Vcc or so. So not enough

    - A pull-up resistor to 2.5V on the input signals could give these a last step.

    - The noise margin (difference between Voh and Vih) will be very small

    - The use of a schmitt trigger input will improve the noise insensitivity

    Good luck, Ton

    PS Maybe the bus hold functionality could be use, but I don't have any experience with it